[NiklasHerrmann21]

Summary

• Prepare release 4.3.0 after the java-security-core split (Feat/split java security core #2032): update all active reactor modules, README dependency examples, integration tests and seven samples.
• Add the dependency updates merged in Build(deps): Bump the prod-deps-ver group across 1 directory with 15 updates #2064 to the existing 4.3.0 changelog.
• Fix a missing legacy Spring Framework version override in the Boot 3 starter: its resolved Spring dependencies now consistently use Boot 3.5.16 / Framework 6.2.19 / Security 6.5.11.
• Add the trusted localhost issuer to the WebFlux sample test configuration so it tests 4.3.0 without disabling issuer validation.

Dependabot review (main only)

• Build(deps): Bump the prod-deps-ver group across 1 directory with 15 updates #2064 reviewed and merged; Spring Boot 3 / Spring Security 6 compatibility pins retained.
• Build(deps-dev): Bump org.springframework:spring-context from 7.0.9 to 7.0.10 in the dev-deps group across 1 directory #2057 closed as superseded by Build(deps): Bump the prod-deps-ver group across 1 directory with 15 updates #2064.
• Build(deps-dev): Bump spring.security.version from 6.5.11 to 7.1.2 #2059 not merged: replaces the Spring Security OAuth2 6.x pin in spring-security-3 with 7.1.2; explanation posted on the PR.
• No changes to main-3.x or its PRs.

Validation (Java 17)

• mvn -B clean install: successful; 1,231 tests, 0 failures/errors, 2 skipped.
• mvn -B verify -Prelease -Dgpg.skip=true: successful; release packaging verified without signing or publishing.
• mvn -B package -f java-security-it/pom.xml: successful; 13 tests.
• All seven CI samples package successfully against 4.3.0 (WebFlux retested after the test configuration fix).
• Resolved dependency trees verified: legacy modules remain on their compatible Spring versions; java-security-core has no runtime Spring or jakarta.servlet dependency.
• git diff --check: successful.

Release scope

Preparation only, as requested: no Maven Central deployment, release tag, or GitHub release created. This PR is not merged automatically.